transmissionof heat ppt

8/12/2019 transmissionof heat ppt

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Volume B Chapter 25Transmission of Heat



Oh no! You will spoil the flaskif you put cold water in it.

After this, the flask cannot be used

to keep water hot anymore.

Myth or Truth?



Why are the handles of saucepans usually made of

plastic?



Why is food

packed in

styrofoam

boxes?



Cooling fins at

the back of

refrigerators.

Why are they

always

painted black?



What we will learn…

•Explain what is meant by conduction,convection & radiation

•The volume & density of a substance willchange during thermal expansion

•Different materials have different ratesof heat flow



Heat is a form

of energy that istransmittedfrom a region of

higher to lowertemperature.



IN 3 WAYS…



Transfer of Thermal Energy

T H E M E T H R E E : T H E R M A L P H Y S I C S

C h a p t e r 7

They are three processes of heat transfer

shown in the pic ture. 1 represents

2 represents

3 represents

ondu tion.

onve tion.

r di tion.



Conduction is transmission of heat through amaterial from the hot regions to the cold regions without any visible movement of the material.



•Solids are better conductors of

heat than liquids and gases as solidparticles are closer together.

•Thus, kinetic energy is transferredmore quickly in solids.

See movie

http://localhost/var/www/apps/conversion/tmp/scratch_4/Conduction%20-particles%20in%20metal%20rod.avi

http://localhost/var/www/apps/conversion/tmp/scratch_4/Conduction%20-particles%20in%20metal%20rod.avi



•When one end of a rod is

heated, particles there

gain energy and vibrate faster .

•These particles collide

with less energeticneighbors.



•Some of their energy istransferred toneighboring particles

which thus gains kineticenergy.

•

This goes on until thecold end reaches sametemperature as hot end

See movie

http://localhost/var/www/apps/conversion/tmp/scratch_4/Conduction-%20pins%20on%20metal%20rod.mpeg

http://localhost/var/www/apps/conversion/tmp/scratch_4/Conduction-%20pins%20on%20metal%20rod.mpeg



bserv tion :

Observe the leng ths of wax that have melted

on the different rods in a given interval of t ime .

WoodIron

Aluminium

Copper



Process:

•When a metal is heated, free electronsgain energy and move faster.

•They move in the spaces between

atoms before colliding with some atomsand transferring energy to them.

Metals conduct heat much faster then

non-metals as they have free electrons.



•Materials conduct heat at different

rates

•Good conductors of heat allow heat to

flow through rapidly

•Bad conductors or good insulators of

heat allow heat to flow through slowly



Good conductors Poor conductors(insulators)

Silver

Copper

Aluminum

BrassIron

Lead

Stainless Steel

Vacuum

Air

Styrofoam

Body fatCork

Water

Glass



1. Uses of Good Conductors•Used where heat has to be

quickly transmitted

E.g. Pans, kettles, cookingutensils

•Usually made of metals like

aluminium, copper, steel



2. Uses of Poor Conductors (Insulators)

• Use where unwanted heat has to be kept

away or to prevent heat loss.

E.g. vacuum flask, handles of saucepan• Made of Cloth, plastics, wood etc

insulator



• Trapped air

between 2 layers

of glass is goodinsulator, reduces

conduction of

heat



.

Brass Wood Aluminium Air Foam Copper

Iron Water Cotton Plastic Mercury Gold

Try This: Fill in the table appropriately

Good Conductors Good Insulators



.

Brass Wood Aluminium Air Foam Copper

Iron Water Cotton Plastic Mercury Gold

ANSWER : Fill in the table appropriately

Good Conductors Good Insulators (orBad Conductors)

Brass Aluminium Wood Air

Copper Iron Foam Water

Mercury Gold Cotton Plastic



Convection is the transmission of heat

through a fluid (liquid or gas) from thehot to the cold region by the circulatingmovement of the fluid.

cool(higher density)

hot

(lower density)

liquid or gas



occurs in fluids (liquids and gases),but not in solids

When one part of fluid is heated, it

expands, becomes less dense andrises

The surrounding cooler and denser

fluid sinks

This produces a circulating movementcalled convection current.



See movie

http://localhost/var/www/apps/conversion/tmp/scratch_4/convection%20current.mpeg

http://localhost/var/www/apps/conversion/tmp/scratch_4/convection%20current.mpeg



•Add potassium permanganate (purple) crystals into a flask of water

•

Heat flask gently just below thecrystals

•Purple-streaked water rises upwards and fan outwards.

•Water becomes evenly purple in

colour after a while.



7.2 Convection

Application 2: Car Radiator• Car engines are kept cool by a

water jacket

• When water in the water

jacket gets heated, it flows

into copper tubes whichinclude many cooling fans.

• A fan causes air to flow past

the tubes and cool water in

them.

• Cooled water flows down andback into engine through a

hose at the bottom.





•Fit two glass chimneys to the top of awooden box with plane glass window

•

Light a candle below one chimney

•Heat from this initiates a convectioncurrent

•Hot air flows out of this chimney

•Cold air flows in through other chimney



Sea and Land Breezes



1. Sea Breeze (Daytime)• In the day, land heats up faster than the sea

•Air above land expands and rises

•Cool air above sea moves in towards theland

•This result in sea breeze



Sea and Land Breezes



2. Land Breeze (Night time)•At night, the land cools faster than the sea

•Warm air above sea rises

•Cool air above land moves out towards thesea

•Result in land breeze



Hot Water System



•Boiler at bottom heats up water andcauses the hot water to rise

•Hot water then moves to the hot waterstorage tank

•Cold water from the cold tank then

moves down to replace the hot water

•A convection current is set up

Hot Water System



Air- Conditioners



•Cold air blown out by the air-conditionersinks because it is denser than warm air.

•Warm air rises and is drawn into air-conditioner to be cooled.

•A convection current is set up

Air- Conditioners





Q1. Which of the following is a good conductor

of heat?

A. Water

B. Oxygen

C. Wood

D. Aluminium



Q2. Why does heat conduction take place faster

in metals?

A. The molecules are larger than non-metals and

this reduces the amount of time taken for

energy to transfer.

B. The presence of free electrons causes heat

energy to transmit quickly.

C.The molecules are packed closer than non-metals and this reduces the time taken for

energy to transfer.

D.The molecules gain energy faster in metals.



Q2. Which of these would you use for the frying

pan and its handle?

A.Pan: Plastic Handle: Copper

B.Pan: Glass Handle: Glass

C.Pan: Copper Handle: Brass

D.Pan: Steel Handle: Wood



Q3. The purpose of using a double-glazed

window is to

A. Save money because glass is expensive.

B. Provide a double protection against harmful

projectiles.

C.Reduce heat conduction by trapping air inside.

D.Improve heat convection by trapping air

inside.



Q4. We need to put on more clothes in winter

because

A. The air is cold and we do not want heat to

escape from our bodies.

B. The clothes is able to trap cold air.

C. The air is cold and we will lose water too

quickly.

D.Winter clothes are nice and trendy.





Q1. Convection takes place because

A. Free electrons make the heat energy travel

faster in the fluid.

B. Hot liquid and cold liquid separatesthemselves from each other.

C. Hot fluid rises and colder fluid sinks.

D. Hot fluid sinks and colder fluid rises.



Q2. The purple color stream is seen to travel

upwards from the center and downwards

from the sides because

A. The heat from the fire causes the

crystal to react with the water.

B. it is the only possible way for thecrystal to dissolve.

C. The water at the top starts to

cool down and water from the

base has to replace it.

D. The heating at the base stirs up a

convection current.



Q3. Convection of heat cannot take place in

A. Alcohol

B. Bricks

C. Nitrogen

D. Paraffin



Q4. We place air conditioners near the ceiling

because

A. air blown from the top will fall to the

ground due to gravity.

B. cold air can be found near the ceiling.

C. hot air rises to the ceiling and cooling it

will create a convection current.

D. cold air rises to the ceiling and cooling

it will create a convection current.



Q5. Sea breeze occurs

A. In the daytime.

B. in the night time.

C. only at midnight.

D. only when it rains.



Radiation is the transmission of heat fromone place to another by means of infra-redradiation which can travel through vacuumor air



Rate of Emitting (giving out) andAbsorbing Heat by Radiation is affectedby:

(a) surface temperature

(b) colour and texture of the surface

(c) surface area



•

Dull black surfaces are good emitters andabsorbers of heat

•Shiny white surfaces are good reflectors

(bad emitter and absorber) of heat



Why does the cork on the dull black surface fall off first?

Ans: Dull black surfaces are good absorbers of heat,

thus it melts the wax faster



Good Emitters Poor EmittersCooling fins at the back

of a refrigerator painted

dull black

A shiny metal teapot and

aluminum foil keep

contents warm



Good Absorbers Poor AbsorbersSolar heating panels are

painted in dull black paint

to absorb heat from sun

Houses in hot countries

are painted in white or

light-coloured paint, to

keep them cooler.

Light colours are chosenfor clothes and cars in hot

weather



Vacuum FlaskPurpose:

• keeps hot liquids hot

and cold liquids cold.

Function:

• Reduces transfer of

heat by conduction,

convection and

radiation





Structure Reduces heat loss/gained by:

vacuum in between

the double glass walls

conduction or convection

Silvered walls

•Outer silvered

surface

•Inner silvered surface

Radiation

•reflects radiation in external

surroundings away from flask

•reflects radiation from hot fluids

back into flask

Foam plastic support conduction

Plastic cap convection





1. State the parts of a vacuum flask and explain what their

functions are.

Structure Purpose (reduces heat

loss/gained by)



They are three processes of heat transfershown in the pic ture. 1 represents

2 represents

3 represents

ondu tion.

onve tion.

r di tion.

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transmissionof heat ppt